A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. A power factor correction (“PFC”)/resonant inductor-inductor-capacitor (“LLC”) power converter includes a power train with a PFC stage followed by a LLC stage. The power converter is coupled to a source of electrical power (an alternating current (“ac”) power source) and provides a direct current (“dc”) output voltage. The PFC stage receives a rectified version of the ac input voltage (from the ac power source) and provides a dc bus voltage. The LLC stage employs the bus voltage to provide the dc output voltage to a load. The power converter including the PFC stage and the LLC stage can be employed to construct an “ac adapter” to provide the dc output voltage to a notebook computer or the like from the ac power source.
Controllers associated with the power converter manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”). Two control processes are often employed to control the output voltage of a power converter formed with the PFC stage followed by the LLC stage. One process controls the bus voltage of the PFC stage to control the output voltage, and the other process controls the switching frequency of the LLC stage to control the output voltage. As will become more apparent, employing two independent processes to control the output voltage of the power converter with the PFC stage and the LLC stage can lead to several design issues that detract from the operation and efficiency of the power converter.
Another area of interest with respect to power converters in general is the detection and operation thereof under light load conditions. Under such conditions, it may be advantageous for the power converter to enter a burst mode of operation. Regarding the burst mode of operation, power loss of a power converter is dependent on gate drive signals for the power switches and other continuing power losses that generally do not vary substantially with the load. These power losses are commonly addressed at very low power levels by using the burst mode of operation wherein the controller is disabled for a period of time (e.g., one second) followed by a short period of high power operation (e.g., 10 milliseconds (“ms”)) to provide a low average output power with low dissipation. The controller as described herein can employ the time interval of the burst mode of operation to estimate an output (or load) power of the power converter.
Accordingly, what is needed in the art is a controller that incorporates a hybrid approach to the control processes for a power converter employing different power stages in a power train thereof to avoid the deficiencies in the prior art. Additionally, what is needed in the art is a controller that can detect and manage a power converter at light loads including an operation of the power converter entering a burst mode of operation to avoid the deficiencies in the prior art.